Mono-ADP-ribosylation is a post-translational modification of proteins in which the ADP-ribose moiety of nicotinamide adenine dinucleotide (NAD) is transferred to a specific amino acid. Several well-characterized mono-ADP-ribosyltransferases have been identified in viruses, bacteria and eukaryotes (Corda and DiGirolamo, EMBO J. 22:1953-1958, 2003). Arginine-specific mono-ADP-ribosyltransferase-1 (ART1) is present on the apical surface of epithelial cells in human airways and is linked to the cell surface by a glycosylphosphatidylinositol (GPI) anchor (Balducci et al., Am. J. Respir. Cell. Mol. Biol. 21:337-346, 1999; Okazaki and Moss, J. Biol. Chem. 273:23617-23620, 1998). ART1 modifies the arginines of several substrates, including defensins such as human neutrophil peptide (HNP)-1, thereby altering their activity (Corda and DiGirolamo Sci. STKE PE53, 2002; DiGirolamo et al., FEBS J. 272:4565-4575, 2005).
Neutrophils, a critical component of the innate immune system, are recruited to airways in response to inflammation or infection. Neutrophil defensins (HNP-1 to -3), stored in azurophilic granules, are small cationic peptides whose main function is to defend the lung against pathogenic micro-organisms (Bals and Hiemstra, Eur. Respir. J. 23:327-333, 2004). High levels of defensins have been found in patients with inflammatory lung diseases such as idiopathic pulmonary fibrosis (IPF) (Mukae et al., Thorax 57:623-628, 2002) and cystic fibrosis (Soong et al., Inflamm. Res. 46:98-102, 1997). In addition to antimicrobial activities and other diverse functions (Rehaume and Hancock, Crit. Rev. Immunol. 28:185-200, 2008), defensins interact with airway epithelial cells, increasing proliferation and stimulating wound repair (van Wetering et al., J. Leukoc. Biol. 77:444-450, 2005). HNPs 1-3 are arginine rich and differ in sequence by one amino acid. The arginines in HNP-1 are critical for maintaining activity of the protein (Zou et al, J. Biol. Chem. 282:19653-19665, 2007).
In vitro, ART1 ADP-ribosylates HNP-1 on arginine 14 with a secondary site on arginine 24. Mono- and di-ADP-ribosylated HNP-1 have previously been isolated from the bronchoalveolar lavage fluid (BALF) of IPF and asthma patients, consistent with a role for the modified HNP-1 in disease (Paone et al., J. Biol. Chem. 281:17054-17060, 2006).